1. Field of Invention
The present invention relates to power compensation techniques used for an alternating current (AC) electrical grid, and more particularly relates to a power compensation apparatus and a power compensation method which are suitable for use in a renewable energy system.
2. Description of Related Art
For an electrical grid, there are two types of electrical power for a power supply load, which includes active power and reactive power. The active power is the electrical power required for maintaining normal operation of a powered device; and, the reactive power is relative abstract, which is the electrical power used in electric and magnetic fields of an electrical circuit for creating and maintaining a magnetic field in an electrical apparatus. Generally, any electrical apparatus with magnetic coils has to consume reactive power for creating a magnetic field, and since no work will be done when the magnetic field is built, the process is refereed to as “no work”.
Currently, with the increasing energy crisis and environmental issues, countries all over the world are vigorously developing renewable energy business, such as wind power generation and solar energy generation. Taking the wind power generation as an example, the installed wind power capacity in China grows rapidly, from a stall-regulated wind power system to a variable-speed and constant-frequency (VSCF) wind power system, and from a wind power system with a gearbox to a direct-drive wind power system without the gearbox. For a stall-regulated wind asynchronous-generator, it needs to absorb lagging reactive power from an electrical grid to create a magnetic field and satisfy magnetic requirements, as well as to compensate reactive power consumed by excitation. However, in common medium or large scale induction machines, the excitation current is about 20%-25% of the rated current, and thus the reactive power required by excitation amounts to 20%-25% of the generator capacity. Thus, the overall reactive power required by an induction generator is about 25%-30% of the generator capacity. This will significantly increase the reactive power load in the electrical grid, resulting in power factor (PF) decrease of the network and voltage decrease and line-loss increase of the network, thereby affecting the stability of the electrical grid. Moreover, the active power outputted by the wind power system varies with the wind speed, and when the active power outputted by a generator is not balanced with the load power of a powered device, frequency of the electrical grid changes and thus endangers the stability of the electric power system.
In the prior art, one solution is to use a static var compensator (SVC). The SVC includes a reactor and a capacitor, wherein switching of the reactor is controlled by a thyristor because the thyristor has a fast response to a control signal, and the on/off switching times also may not be limited. When the voltage fluctuates, the SVC can adjust the voltage rapidly and smoothly, so as to satisfy the requirements of dynamic reactive power compensation. However, when the thyristor is used to control the switching of the reactor, high-order harmonics is generated, and thus a special filter is needed.
In view of this, it is a problem desired to be solved by this industry regarding how to design a new power compensation technical solution which may not only provide reactive power for controlling the electrical grid voltage, but also achieve a balance between output power and load power when the system is loaded with a balanced or un balanced load.